There is no predefined shortcut for Highlight but you can easily assign one:
Select Tools > Customize and click the Keyboard button.
Below Categories, select "All Commands".
Below Commands, select "Highlight".
Enter the desired shortcut.
In order to make the shortcut available to all documents, select Normal.dot
in the "Save changes in" list.
Click Assign.
Click Close.

To save the change to Normal.dot, press and hold the Shift key while you
select File > Save All. If a dialog box opens, asking whether to save changes
to Normal.dot, click Yes.

The shortcut will apply the highlight color that is currently selected in
the Highlight icon in the Formatting toolbar.